Radio communication networks were originally developed primarily to provide voice services over circuit-switched networks. The introduction of packet-switched bearers in, for example, the so-called 2.5G and 3G networks enabled network operators to provide data services as well as voice services. Eventually, network architectures will likely evolve toward all Internet Protocol (IP) networks which provide both voice and data services. However, network operators have a substantial investment in existing infrastructures and would, therefore, typically prefer to migrate gradually to all IP network architectures in order to allow them to extract sufficient value from their investment in existing infrastructures. Also to provide the capabilities needed to support next generation radio communication applications, while at the same time using legacy infrastructure, network operators could deploy hybrid networks wherein a next generation radio communication system is overlaid onto an existing circuit-switched or packet-switched network as a first step in the transition to an all IP-based network. Alternatively, a radio communication system can evolve from one generation to the next while still providing backward compatibility for legacy equipment.
One example of such an evolved network is based upon the Universal Mobile Telephone System (UMTS) which is an existing third generation (3G) radio communication system that is evolving into High Speed Packet Access (HSPA) technology. Yet another alternative is the introduction of a new air interface technology within the UMTS framework, e.g., the so-called Long Term Evolution (LTE) technology. Target performance goals for LTE systems include, for example, support for 200 active calls per 5 MHz cell and sub 5 ms latency for small IP packets. Each new generation, or partial generation, of mobile communication systems add complexity and abilities to mobile communication systems and this can be expected to continue with either enhancements to proposed systems or completely new systems in the future.
LTE uses orthogonal frequency division multiplexing (OFDM) in the downlink and discrete Fourier transform (DFT)-spread OFDM in the uplink. The basic LTE downlink physical resource can thus be seen as a time-frequency grid as illustrated in FIG. 1, where each resource element corresponds to one OFDM subcarrier during one OFDM symbol interval. In the time domain, LTE downlink transmissions are organized into radio frames of 10 ms, each radio frame consisting of ten equally-sized subframes of length Tsubframe=1 ms as shown in FIG. 2.
Furthermore, the resource allocation in LTE is typically described in terms of resource blocks, where a resource block corresponds to one slot (0.5 ms) in the time domain and 12 contiguous subcarriers in the frequency domain. Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth. Downlink transmissions are dynamically scheduled, i.e., in each subframe the base station (typically referred to as an eNB in LTE) transmits control information indicating to which terminals and on which resource blocks the data is transmitted during the current downlink subframe. This control signalling is typically transmitted in the first 1, 2, 3 or 4 OFDM symbols in each subframe. A downlink system with 3 OFDM symbols as the control region is illustrated in FIG. 3.
On the LTE uplink, single-carrier frequency division multiple access (SC-FDMA) is used in a manner wherein, as much as possible, the structure is aligned as much as possible with the LTE downlink. Thus, as shown in FIG. 4(a), the uplink subcarrier spacing in the frequency domain is also 15 kHz and resource blocks having 12 subcarriers are also defined for the LTE uplink. An example of the LTE uplink subframe and slot structure is shown as FIG. 4(b). Therein, it can be seen that one subframe includes two equally sized slots, each slot having six or seven SC-FDMA blocks (normal and extended cyclic prefix, respectively). An example of LTE uplink resource allocation is provided in FIG. 4(c), wherein the assigned uplink resource for a user corresponds to the same set of subcarriers in the two slots.
The 3GPP LTE radio access standard has been written in order to support high bitrates and low latency both for uplink and downlink traffic. All data transmission is in LTE controlled by the radio base station. In order to support efficient uplink scheduling, a method has been defined to inform the base station of the buffer status of the UE. This method mainly consists of buffer status reports (BSR) and scheduling requests (SR). A number of rules have been defined regarding when a UE should trigger a BSR, such as arrival of new data to an empty buffer. The BSR is sent on the physical uplink shared channel (PUCCH) like other data transmissions.
A BSR transmission therefore requires a valid uplink resource. SR has been defined as single bit information indicating to the base station that a BSR has been triggered in the UE. The SR can be transmitted either on a preconfigured semi-static configured periodic resource on the physical uplink control channel (PUCCH), referred to as D-SR, or if no such resource has been configured, on the Random Access Channel (RACH), referred to as RA-SR. The D-SR resource on the PUCCH uses a code division multiple access scheme to uniquely identify the user on a specific time/frequency resource.
On each LTE uplink resource block pair dedicated for PUCCH, up to 36 unique code resources is available. A resource block pair is a time-frequency resource consisting of two in time consecutive resource blocks made up from one slot (0.5 ms) in time and 180 kHz in frequency. Two slots make up a transmission time instance (TTI). It is up to the LTE base station, i.e., an eNodeB, to divide the resources in time, frequency and code, where the trade-off stands between short periodicities giving low latency but costing in larger overhead for control channels versus lower overhead but with longer delay.
Thus, as described above, in LTE there is a semi-static configuration of SR resources and no SR collisions since the SR resources are not reused. For an active session the static configuration can limit the latency and the available bandwidth of the user. A remedy would be to configure a shorter SR periodicity, but this would limit the number of users that could be present in the system. Consider, as a purely illustrative example, that a system was configured to use 2 resource blocks for SR and that the UEs has a 1 ms SR periodicity, this would limit the system to 72 UEs with a SR resource.
The reason for the limit in latency is the corresponding waiting periods for requesting UL resources. The need for uplink resources can be either that the UE wishes to send data, but it can also be feedback to higher layers. One scenario where faster feedback is important is TCP-slowstart. This a mechanism to sense the bandwidth to user by slowly raising the transmission bandwidth, while waiting for confirmation. The faster the UE can give feedback, the faster the transmission will rise. The extra latency on feedback can also be interpreted by higher layers as a limit in downlink bandwidth. This can lead to throttling of the downlink throughput, even if the LTE system could support higher bandwidth.
Accordingly, it would be desirable to provide a mechanism to reuse resources for transmitting SRs, while also being able to identify users and resolve SR collisions, e.g., on the PUCCH in an LTE system.